The meanings of certain acronyms and abbreviations used herein are given in Table 1.
TABLE 1Acronyms and AbbreviationsECGElectrocardiogramPIUPatient Interface UnitRFRadiofrequency
Medical catheterizations are routinely carried out today. For example, in cases of cardiac arrhythmias, such as atrial fibrillation, which occur when regions of cardiac tissue abnormally conduct electric signals. Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy, e.g., radiofrequency energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions.
A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure. Commonly assigned application Ser. No. 13/339,782 (now U.S. Pat No. 8,956,353), which is herein incorporated by reference, discloses the use of an irrigation pump to cause irrigation fluid to flow through a lumen of the catheter in order to cool the ablation site.
A typical catheterization system includes a catheter which is inserted through a patient's vascular system into a chamber or vascular structure of the heart. The catheter's distal tip is brought into contact with the heart wall for obtaining electrical and positional information that is processed by a console that includes a processor for generating activation maps, anatomical positional information and other functional images. The system typically includes an electrocardiogram (ECG) monitor coupled to receive signals from one or more body surface electrodes. The ECG signal is typically received through an interface with the console, e.g., a patient interface unit having an analog input and an isolated ground may be used to provide an ECG synchronization signal to the console.
An electrically conductive fluid, e.g., saline, is delivered through a lumen in the catheter from a reservoir via a hydraulic line. The lumen terminates in exit pores through which the liquids emerge to cool an ablating electrode at the distal portion of the catheter and also the tissue ablation site. A peristaltic pump is connected to the hydraulic line and causes the fluid to be delivered to the catheter at a desired rate. One difficulty with such an arrangement is that operation of equipment in the environment, e.g., the pump produces electrical effects, which produce noise that can be picked up by the hydraulic line and can interfere with the analysis and display of the intracardiac ECG on the monitor. The electrical emissions or signals are usually observed in ECG leads connected to a patient who is being transfused or infused with the electrically conductive solution. Any currents that flow in the patient's body as a result of this potential are sensed as characteristic noise added to the ECG signals.
This noise has been observed in patients connected to a peristaltic pump for cardiac assist, dialysis treatments and irrigation of an ablation catheter used in treating cardiac arrhythmias. Many sources have been proposed as sources for the noise, some focusing on the pump itself.
Without being bound by any particular theory, the following discussion as set forth in U.S. patent application Ser. No. 13/327,448 (now U.S. Pat. No. 9,101,269), filed Dec. 15, 2001, entitled ELECTROGRAM NOISE REDUCTION, the entire content of which is incorporated herein by reference, is offered to facilitate understanding of the various embodiments described and disclosed herein: In one respect the hydraulic line may function as a receiving antenna that collects noise from the surrounding environment and may constitutes one source of the noise. In another respect, the pump may be another source of electrical noise, created by a triboelectric effect, whereby an induced charge is created on the surface of flexible tubing used in the pump and on the surface of the rotor surfaces used to compress the tubing. The rubbing or deforming action of the rotor against the tubing surface displaces electrical charge. Some of the charge is collected on the rotor and some is collected on the tubing surface. The tubing wall is generally an insulator, so that the external charge on the outside surface of the tube is induced on the inside of the tubing bore if the fluid in the tubing is an electrical conductor. In consequence, a generator potential appears between the electrically conductive fluid and the pump rotor. Any electrical circuit connecting these two points allows current to flow. Such current, if sensed or intercepted by the EKG circuitry, produces undesirable signals on the EKG tracing that are perceived as “ECG noise” by the operator. Because the triboelectric potential appears in series with the capacitance of the external and internal tubing walls, which are generally insulators (plastic), the triboelectric current has bursty characteristics.
Additionally or alternatively, the observed current may arise from a piezoelectric effect in the tubing walls. Further additionally or alternatively, there appears to be a strong amplification mechanism resulting from the motion of the tubing walls as they are squeezed between the rotor rollers and the pump race, causing a dynamic change in tubing capacitance, which is in series with the triboelectric charge.
The noise, as observed on intracardiac ECG recordings, appears as spikes, making the ECG signals difficult to interpret, and these spikes (typically ranging between about 0.05 mV and 0.2 mV) can even be confused as ECG waves themselves. Additionally, a fast Fourier transform applied to the noise to obtain its power spectrum finds component sinusoids at repetition frequencies equal to the impact rate of the rotor rollers (N) on the tubing surface along with higher harmonics. The repetition frequencies are dependent on the number of rollers in a rotor, and are to be distinguished from the rotor rotation rate itself.
FIGS. 8A-8C illustrate actual ECG recordings with repetitive “spikes” (designated by arrows) in intracardiac ECG signals during ablation procedures using SmartAblate Pump. Analyzing the recordings, it was determined that the frequency of the spikes is proportional to the speed of pump motor (or proportional to the flow rate), such that, for example, for 30 ml/min the spikes occur at about 85 ms time intervals, and for 15 ml/sec the spikes occur at about 170 ms (double the 85 ms time interval). It also appeared that the amplitude of the spikes increased with flow rate (though no linearly), such that the effect was clearly observed only for high flow rates and was indistinguishable for low rates used during mapping/navigation phases. The reported amplitudes of the spikes (measured peak-to-peak) were in the range of about 100-200 μV. It is understood that the noise differs for different pump designs. Time between peaks and peak-to-peak voltage can vary.
Treatments to reduce the noise have included lining the pump roller and pump bed, coating the pump hydraulic line with an antistatic chemical, and/or wetting the contact surfaces of these components. However, the reduction tends to be insignificant and/or temporary.
The aforementioned U.S. patent application Ser. No. 13/327,448 (now U.S. Pat. No. 9,101,269) describes a hydraulic line having an outer portion coated with a material or an antistatic chemical, including the portion contacting the outer surface with the rotating element of the pump. The material contains liquid water and an ionic surfactant. The antistatic chemical may be selected from the group consisting of soap water, saline and water. In addition, the contacting portion of an outer surface of the hydraulic line may be coated with an electrical conductor, for example, indium tin oxide or aluminum foil. The hydraulic line may also be impregnated with the anti-static chemical.
The aforementioned U.S. patent application Ser. No. 131327,448 (now U.S. Pat. No. 9,101,269) also describes a system wherein a catheter has a lumen for passing an electrically conductive fluid therethrough to exit the catheter at its distal portion, the lumen connectable to an irrigation pump to form a fluid communication therewith. A fluid reservoir is connected to the lumen for supplying the electrically conductive fluid to the catheter. Electrocardiogram circuitry is connectable to the subject for monitoring electrical activity in the heart. An electrically conductive cable links the electrically conductive fluid to an electrode that is in contact with the subject. According to an aspect of the system, the catheter has mapping electrodes disposed on the distal portion and the electrode is located on the catheter proximal to the mapping electrodes. According to a further aspect of the system, the electrode is located on a second catheter that is introduced into the subject. According to one aspect of the system, the catheter has an inlet port, and a connector electrically contacts the electrically conductive fluid at the inlet port, and connects the electrically conductive fluid to a patient ground. According to another aspect of the system, the electrically conductive cable is electrically connected to the electrically conductive fluid downstream of the irrigation pump. According to an additional aspect of the system, the electrically conductive cable is a metallically shielded cable.
However, the use of an additional external connection cable increases the burden on an electrophysiology professional by a typical catheterization system which already employs numerous connectors and cables to and from equipment pieces and the patient. Moreover, the use of a cable that links the electrically conductive fluid to an electrode that is in contact with the patient may render the system's ability to reduce ECG noise dependent on a number of factors, including the quality of the connection between the electrode and patient, the location of the electrode, and the impedance of the patient's body, which differs from patient to patient. In addition, any added or modified electrical link within the catheterization system may subvert the equipment grounding conductor paths necessary for the system circuit to meet safety requirements.
Accordingly, there is a desire for a catheterization system that reduces or eliminates ECG noise. There is a desire that the noise reduction or elimination be accomplished without compromising patient safety or regard to factors, including the quality of the connection between the electrodes and patient, the location of the electrode on the patient, and the impedance of the patient's body, which differs from patient to patient. There is also a desire for a catheterization system which avoids the use of any additional lengthy cable, especially one that extends between the patient and the fluid source or fluid pump which can tangle or disrupt workflow of the attending medical professionals.